The concept of reinforcing thermoplastic polymers or elastomers has produced an almost endless field of formulations. The most often used reinforcing polymer has been glass fiber, although higher performance fibers such as carbon and aramid have been gaining increased acceptance. Mineral reinforcements, although often regarded only as fillers or extenders, can improve certain properties of the base polymer. Reinforcers increase tensile strength when applied to a base resin and both reinforcers and fillers increase flexural modulus, with reinforcers offering greater increases. Impact properties in general will be increased by reinforcers. Both fillers and reinforcers will improve thermal properties, again with reinforcers offering greater increases.
Both reinforcers and fillers will lower the shrinkage of thermoplastics and elastomers thereby giving a more consistent material, however, reinforced thermoplastics shrink less in the direction of the flow than they do in the perpendicular direction. This property is termed anisotropic shrinkage.
It has been shown that there is minimum fiber length that would approach the degree of reinforcement afforded with a continuous system and yet not overly interfere with the moldability of the thermoplastic or elastomeric resin. One very important aspect of fiber size is the fact that laboratory tests of pressurized extrusion of a flexible thermoplastic between the outer strands of a rope have proven that for cross sections or wall thicknesses of the plastic that are less than 0.050 inch (1.27 mm) thick, "one dimension" reinforcement is approached because the thickness is less than the fiber length, creating forced alignment along the injection axis. Almost all the fibers are aligned in the flow direction giving 95% of the maximum reinforcement. This flow direction is perpendicular to the rope axis between the outer strands and then parallels said rope axis upon the plastic contacting the rope core. However, where wall thickness runs between 0.050 inch (1.27 mm) and 0.250 inch (6.35 mm) planar rather than one dimensional reinforcement is attained. This implies that half of the fibers are aligned in one direction and the other half are aligned in the perpendicular direction resulting in 50% of the maximum reinforcement obtained when all the fibers are aligned in direction of flow.
Tests on certain thermoplastics have shown that the tensile strength of the resin has been increased by more than 50% from 10,000 PSI to 16,000 PSI (704-1127 Kg/cm.sup.2) by the addition of 5% fiberglass. Likewise, when the percent reinforcement has been increased to 5% or greater the Izod impact strength is increased by 50%; the flexural strength is increased by 22% and the flexural modulus is increased by 100%.